I think you have answered your own question. ie I agree with what you have said and think.
I might add that by taking out some of the 8" long crappie in that lake you may help the rest to grow biggers.
Sounds like they are all from the same year class or spawn.
It appears that the lake does not have the carrying capacity (food supply) to support faster growth rates. The lake needs to increase the food supply and habitat or water quality or reduce the number of fish so that those left can grow faster.
Only way a fish can grow is to take in more energy (food) than it has to expend (energy) to catch it's prey. Also the further south the lake that longer the growing season and the shorter the winter cold season. Some fish need warmer water to grow faster. Crappie is one of them. Some other fish grow best in colder water. Northern Pike would be an example of the latter speices.
I fish a lake that is full of small crappie. You can catch small crappie almost anywhere on this small lake. But finding a crappie longer than 12" is difficult.